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"sewer" smell from the ductwork

I have a customer complaining of a "sewer" smell coming from the ductwork. The smell only seems to be noticeable when the aux heat strip is on. The air handler is an older Trane unit with 10kw electric heat strip. There doesn't seem to be an odor when only Heat pump is engaged or when the fan is operated, only when the heat strip is on. The exterior of the ductwork seems to be OK when visually inspected. I haven't cut into it yet to check the interior. The odor seems more intense the longer the aux heat is on. Any ideas.

No, just standard ductboard. The heat strip was visually inspected and looks and operates normally. The customer says "sewer" smell, I have been unable to smell it on two previous visits, however, an electrician said it made his eyes water.

I can't say I have ever heard of a sewer smell with ductboard but during a geothermal install everyone was getting sick prior to and during the removal of all ductboard in the residence as well as a smell that was not mold.

Heard of it, but never ran into it. I thought it was more prevalent in the humid south, and more commonly associated with high eff. equipment. The unit is about 20 years old, so replacing just the coil is not practical. I just looked at a Youtube video of a guy who claims to have solved the problem with a Pine-Sol solution sprayed on the coil. The aux heat issue has me stumped though, the smell should be most noticeable at initial fan start-up, not after the kick-on of the aux heat strip. If it is the evaporator coil, the smell should also be present in cooling mode. The problem is its about 5*F outside, not a good time to switch to cooling mode.